/Engine/Source/Runtime/CoreUObject/Public/UObject/PackageResourceManager.h
TODO: This call should not hit the disk/network or block, but it currently does, to find the extension. This call will always return a non-null handle, even if the package does not exist
AnFOpenAsyncPackageResult, with Handle that will read from the package if it exists, or will be in the canceled state if the package does not exist, and with other data describing the returned archive (seeFOpenAsyncPackageResult)
[FOpenAsyncPackageResult](API\Runtime\CoreUObject\UObject\FOpenAsyncPackageResult) OpenAsyncReadExternalResource ( EPackageExternalResource ResourceType, FStringView Identifier )